The phrase is a Hebrew idiom, well understood in the culture of Yeshua and the apostles.
It is a reference to Yom Teruah aka Rosh Hashanah.
Idioms do not translate at all; or at least very poorly. So many cannot figure this out.
But the context is the return of Christ :
Mark 13:28-37New American Standard Bible (NASB)
28 “Now learn the parable from the fig tree: when its branch has already become tender and puts forth its leaves, you know that summer is near.
29 Even so, you too, when you see these things happening recognize that He is near, right at the door.
30 Truly I say to you, this generation will not pass away until all these things take place.
31 Heaven and earth will pass away, but My words will not pass away.
32 But of that day or hour no one knows, not even the angels in heaven, nor the Son, but the Father alone.
33 “Take heed, keep on the alert; for you do not know when the appointed time will come.
34 It is like a man away on a journey, who upon leaving his house and putting his slaves in charge, assigning to each one his task, also commanded the doorkeeper to stay on the alert.
35 Therefore, be on the alert—for you do not know when the master of the house is coming, whether in the evening, at midnight, or when the rooster crows, or in the morning—
36 in case he should come suddenly and find you asleep.
37 What I say to you I say to all, Be on the alert!’”